1. Field of the Invention
The present invention relates to the production of pyromellitic acid and pyromellitic anhydride. These compounds are usable in a wide variety of applications, for example, the production of various resins such as polyimide for electronic materials, specialty plasticizers and epoxy hardeners.
2. Description of the Prior Art
Japanese Patent Application Laid-Open No. 56-26839 discloses a method of producing pyromellitic acid by a liquid-phase oxidation of 2,4,5-trimethylbenzaldehyde (hereinafter referred to as “TBAL”) by air in the presence of a catalyst comprising a bromine compound and a heavy metal. Japanese Patent Application Laid-Open No. 58-121244 proposes to reduce catalyst costs by separating the aqueous oxidation solution into crude crystals of pyromellitic acid and a mother liquor and recycling the separated mother liquor into the oxidation reactor. The crude crystals of pyromellitic acid obtained by the oxidation in a water solvent can be purified by the recrystallization from water. However, since the recrystallization mother liquor dissolves a part of the produced pyromellitic acid and the heavy metal salt thereof, the recrystallization yield becomes lower and the catalyst costs becomes larger. The discharge of all the liquids separated from the production steps as the process waste water imposes a very large burden on the production method and economy. If the liquid as separated is reused as a part of the solvent for the oxidation, the activity of the oxidation catalyst is reduced because of excessive dilution to reduce the oxidation efficiency.